Specifically, if you place two uncharged metal plates side by side in a vacuum, they will move towards each other, seemingly without reason. They won’t move a lot, mind. Two plates with an area of a square metre placed one-thousandth of a millimetre apart will feel a force equivalent to just over a tenth of a gram.

The Dutch physicist Hendrik Casimir first noted this minuscule movement in 1948. “The Casimir effect is a manifestation of the quantum weirdness of the microscopic world,” says physicist Steve Lamoreaux of Yale University.

It has to do with the quantum quirk known as Heisenberg’s uncertainty principle, which essentially says the more we know about some things in the quantum world, the less we know about others. You can’t, for instance, deduce the exact position and momentum of a particle simultaneously. The more certain we are of where a particle is, the less certain we are of where it is heading.

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A similar uncertainty relation exists between energy and time, with a dramatic consequence. If space were ever truly empty, it would contain exactly zero energy at a precisely defined moment in time – something the uncertainty principle forbids us from knowing.

It follows that there is no such thing as a vacuum. According to quantum field theory, empty space is actually fizzing with short-lived stuff that appears, looks around a bit, decides it doesn’t …